Q: iCloud backup failing

This is just a starting point. Force restart your phone and try again. No data is lost with this procedure. Hold down on the sleep and home buttons at the same time for about 10 seconds until the Apple logo appears on the screen.

This might help.

If iCloud Backup couldn’t be completed or you can’t restore - Apple Support

check if you are connected with wifi and also check if your free space is enough to back up

ok so the best thing to do is back up your old phone using itunes then connect your new phone then restore it! disable first both your find my iphone

Hi

Go to itunes main page sign out then sign back in .

Do the same with your iCloud account sign out then sign back in .

If this doesn't work leave it try again tomorrow .

Cheers

Brian